Current non-contact radar sensing technology is generally based on the Doppler radar sensing principle, and implemented in continuous-wave in direct conversion or Self-Injection Phase-Locked signal for sensing in transmitting and receiving of the radar. In the former way, a continuous wave being a radar source is used for detection. The continuous wave is transmitted to a target object, and a back scattering signal reflected from the target object is analyzed to detect the target object. It is understood that motion of the object will make the back scattering signal create the Doppler frequency/phasing angle modulation. Therefore, to use a direct conversion receiver removes the continuous wave leakage of the transmitter and the carrier of the back scattering signal, and the motion of the object can be measured. In the latter way, the detecting wave of the monitoring system is a continuous wave and is generated by an injection phased-locked oscillator. Likewise, the continuous wave is transmitted to propagate to an object, which causes Doppler frequency/phasing angle modulation of a back scattering signal. The back scattering signal is then propagated back into the injection phase-locked oscillator, and a motion of the object can be measured based on the self-injection locked property of the injection phased-locked oscillator.
Since the distance for sensing an object depends on the intensity of the wireless signal, intensity of the wireless signal needs to be increased for longer distance detection. Generally, stronger intensity of wireless signal needs higher power of electromagnetic wave, and the higher power causes more energy consumption. In addition, an adverse effect might be caused for the detected object.